1. Field of Invention
The present invention relates generally to laminated piezoelectric ceramic transformers for power transfer circuits. More specifically, the present invention relates to a laminated piezoelectric transformer having a symmetric structure consisting of two multi-layer input sections which are bonded to on the upper and lower surfaces of a centrally poled output section, as well as electrode configurations for efficiently tapping the input and output portions.
2. Description of the Prior Art
Wire wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions, fluorescent lamp ballasts, CCFL backlighting, and others. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce. In addition to being large in size and weight, wound transformers create EMI which can disrupt the performance of other circuits and components in proximity to the transformer, which is a major issue in compact portable devices having a multitude of circuitry in a small packing area, such as laptop computers, PDA's, camcorders, and other handheld devices. Furthermore, in view of high frequency applications and compact size application, the electromagnetic transformer has many disadvantages related to the materials used in their manufacturing. Magnetic materials used for the cores of transformers have two types of electrical losses, eddy current loss due to finite electrical conductivity and hysteresis (magnetic) loss. A third type of loss is related to the windings of the transformer. These windings are made from copper wire, which copper losses include not only DC resistance loss, and additional ohmic loss caused by non-uniform current density concentrations arising from the proximity effect and skin effect. These losses, specifically hysteresis and skin effect losses increase in high frequency applications and force the designing engineer to over-design the magnetic components which, in turn, affects the final size. Furthermore,-wire-wound transformers also require winding isolation material, which also affects the final size of the component. This is even a bigger issue in high voltage transformers where dielectric breakdown risk between high voltage and low voltage wiring limits the minimum thickness of the isolation material used. Furthermore, the maximum permissible temperature of a transformer is approximately 100° C. and is limited by both magnetic material and winding isolation material considerations. This temperature limit along with the surface-to-ambient thermal resistance of the component limit the average power dissipation density (W/cm3) in the component. This power dissipation density limit translates into a maximum current density limit in the copper winding and a maximum peak AC flux density in the core material, and thus in the maximum power density that the wire-wound transformer can supply.
To remedy this and many other problems of wire-wound transformers, piezoelectric ceramic transformers (or PTs) utilizing the piezoelectric effect have been provided in the prior art. In contrast to electromagnetic transformers, PTs have a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and dimensions and shape of materials of construction of the transformer including the piezoelectric ceramics and electrodes. Furthermore, PTs have a number of advantages over general electromagnetic transformers. The size of PTs can be made much smaller than electromagnetic transformers of comparable transformation ratio, PTs are nonflammable, and produce no electromagnetically induced noise.
Piezoelectric transformer technology has evolved around three fundamentally different PT families: “Rosen-type” PTs, “Thickness-type” PTs, and “Laminated-type” PTs. Rosen-type PTs were the first PTs developed and are characterized by a common area for the input and the output section corresponding to the transversal area of the ceramic body. This area is typically transverse to the propagation direction of the acoustic standing front-wave. Furthermore, the input to output coupling area is also, typically, the nodal area of the PT, i.e., the area with no deformation and higher stress levels. Rosen-PTs have been proposed in rectangular, circular, or annular shapes. Thickness-type PTs make use of discs or plates vibrating in the thickness mode. In these PTs, the coupling areas between the input and the output are the major surfaces of the input and output sections. In these PTs, the nodal point is established in the coupling area. In laminated-type PTs the input and output are also acoustically coupled at their major surfaces. However, in these types of PTs the nodal point does not separate the input from the output section. The coupling area between input and output is NOT a nodal area of the PT, but it moves with the vibration of the PT.
The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type PT comprises a flat ceramic slab 20 appreciably longer than it is wide and substantially wider than it is thick. Rosen-type PTs have been proposed in various forms and configurations, including rings, flat slabs and the like as disclosed in U.S. Pat. No. 2,830,274 (1958) by C. Rosen el al., U.S. Pat. No. 3,562,792 (1971) and U.S. Pat. No. 3,764,848 (1973) both by D.Berlincourt, U.S. Pat. No. 4,767,967 (1988) by Tanaka et al, U.S. Pat. No. 5,736,807 by Hakamata et al., and others. Typical examples of a prior Rosen-type PTs are illustrated in FIGS. 1 and 2. In the case of FIG. 1, the piezoelectric body 20 is in the form of a flat slab that is considerably wider than it is thick, and having greater length than width.
As shown in FIG. 1, a piezoelectric body 20 is employed having some portions polarized differently from others. A substantial portion of the slab 20, the generator portion 22 to the right of the center of the slab is polarized longitudinally, and has a high impedance in the direction of polarization. The remainder of the slab, the vibrator portion 21 is polarized transversely to the plane of the face of the slab (in the thickness direction) and has a low impedance in the direction of polarization. In this case the vibrator portion 21 of the slab is actually divided into two portions. The first portion 24 of the vibrator portion 21 is polarized transversely in one direction, and the second portion 26 of the vibrator portion 21 is also polarized transversely but in the direction opposite to that of the polarization in the first portion 24 of the vibrator portion 21.
In order that electrical voltages may be related to mechanical stress in the slab 20, electrodes are provided. If desired, there may be a common electrode 28, shown as grounded. For the primary connection and for relating voltages at opposite faces of the low impedance vibrator portion 21 of the slab 20, there is an electrode 30 opposite the common electrode 28. For relating voltages to stresses generated in the longitudinal direction in the high impedance generator portion 22 of the slab 20, there is a secondary or high-voltage electrode 35 on the end of the slab for cooperating with the common electrode 28. The electrode 35 is shown as connected to a terminal 34 of an output load 36 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 28 and 30 of the low impedance vibrator portion 21 is stepped up to a higher voltage between the electrodes 28 and 35 in the high impedance generator portion for supplying the load 36 at a much higher voltage than that applied between the electrodes 28 and 30. The applied voltage causes a deformation of the slab through proportionate changes in the x-y and y-z surface areas. More specifically, the Rosen PT is operated by applying alternating voltage to the drive electrodes 28 and 30, respectively. A longitudinal vibration is thereby excited in the low impedance vibrator portion 21 in the transverse effect mode (d31 mode). The transverse effect mode vibration in the low impedance vibrator portion 21 in turn excites a vibration in the high impedance generator portion 22 in a longitudinal effect longitudinal vibration mode (g33 mode). As the result, high voltage output is obtained between electrode 28 and 35. Typically, Rosen-type PTs are designed to operate under half wavelength (lambda/2) or three half wavelength (3×lambda/2). The total length of the PT of FIG. 1 determine the value of the operational resonance frequency of the PT.
An inherent problem of such prior Rosen-PTs is that they have relatively low power transmission capacity. This disadvantage of prior PTs relates to the fact that little or no mechanical advantage is realized between the vibrator portion 21 of the device and the driver portion 22 of the device. Because the driver and vibrator portions each is intrinsically a part of the same electroactive member, the transmission of energy between portions is limited to the transverse area of the longitudinal body. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices.
A second problem of Rosen-PTs, such the one of FIG. 1, is the non-symmetric structure in the length direction. Since the polarization direction in ceramic piezoelectric materials relies significantly on the material properties, such as in the stiffness, dielectric permitivity, and piezoelectric properties, the mechanical behavior of the Rosen-PT will not be mechanically symmetrical in the length direction. As a result, Rosen-PTs show spurious bending resonances around the main resonance frequency, specifically when thin bodies are used. This bending resonance may interfere with the main resonance of the PT and thus diminish the efficiency of the PTs. Additionally, the spurious bending resonance may affect the tracking circuitry of the Rosen-PT and may render the PT useless in practice.
Additionally, since the transmitted power density is limited by the strain endurance of the piezoelectric material, Rosen-type PTs are limited in power to the maximum permissible tensile stresses in the nodal transversal area, which is typically very small. As consequence of this, Rosen-PTs become mechanically weak and may suffer fracture in the nodal transversal area.
Another problem with prior Rosen PTs is that the input and output capacitances depend upon the total dimension of the ceramic bar used. Once the dimensions of the slab are selected, the value of the output capacitance design is fixed since it depends on the thickness of the bar and the half of the total length of the bar for Rosen-type PT operating in the lambda-half mode.
Another drawback of conventional Rosen-type PTs is that since the electrode of the high voltage section is located in the loop of vibration, i.e., in the vibrating direction, connection of the external terminals adversely affects vibration or largely degrades reliability.
The second family of PTs is the “Thickness-type PT”, which are PTs polarized and vibrating in the thickness direction (i.e., vibrations are parallel to the direction of polarization of the layers). Illustrative of such thickness mode vibration PTs is the device of U.S. Pat. No. 5,118,982 to Inoue shown in FIG. 3. A thickness mode vibration PT typically comprises a low impedance portion 11 and a high impedance portion 12 stacked on each other. The low impedance portion 11 and the high impedance portion 12 of the thickness mode PT typically comprises a series of laminate layers of ceramic alternating with electrode layers. Each portion is composed of at least two electrode layers and at least one piezoelectric material layer. Each of the piezoelectric ceramic layers of the low impedance portion 11 and the ceramic layer of the high impedance portion 12 are polarized in the thickness direction (perpendicular to the plane of the interface between the ceramic layers). Every alternate electrode layer in each portion 11 or 12 may be connected to each other and to selected external terminals.
The thickness mode PT of FIG. 3 comprises a low impedance vibrator portion 11 including a plurality of piezoelectric layers 211 through 214 and a high impedance vibrator portion 12 including a piezoelectric layer 222, each of the layers being integrally laminated, and caused to vibrate in thickness-extensional mode. The low impedance portion 11 has a laminated structure which comprises multi-layered piezoelectric layers 211 through 214 each being interposed between electrodes including the top surface electrode layer 201 and internal electrode layers 231 through 234. The high impedance portion 12 is constructed of the bottom electrode layer 202, an internal electrode layer 234 and a single piezoelectric layer 122 interposed between both electrode layers 202 and 234. Polarization in each piezoelectric layer is, as indicated by arrows, in the direction of thickness, respectively. In the low impedance portion 11, alternating piezoelectric layers are polarized in opposite directions to each other. The polarization in the high impedance portion 12 is also in the direction of thickness. The PT has a common electrode 234 to which one terminal 16 of each portion is connected. The total thickness of the PT of FIG. 3 is restricted to a half wavelength (lambda/2) or one full wavelength (lambda) of the drive frequency.
When an alternating voltage is applied to the electrode layers across the ceramic layer of the vibrator portion 11, a vibration is excited in the ceramic parallel to the direction of the polarization of the layers in the longitudinal vibration mode (d33 mode). This vibration of the low impedance portion 11 excites a vibration (g33 mode) in the high impedance portion 12. As the high impedance portion 12 vibrates, the g33 mode deformation of the high impedance portion 12 generates an electrical voltage across the electrodes of the high impedance portion 12. When operating the PT in the thickness-extensional mode with a resonance of lambda/2 mode (both end free fundamental mode) or lambda mode (both end-free secondary mode), the PT may operate in a frequency range of 1–10 MHz.
Referring now to FIG. 4: It is characteristic of PTs that they preferably vibrate in a resonant mode predominantly along one plane or direction (i.e., radial or longitudinal planes, and thickness or longitudinal directions). A problem occurs with PTs when the ratio between the longitudinal or radial dimension is close to the thickness dimension. When the ratio between the height H and the radius R of the PT are close to unity, then radial or longitudinal mode resonant frequency and thickness mode resonant frequency are also close to each other. When the resonant frequencies are very close to each other, then the vibrations interfere with each other. This leads to aberrant vibrational modes that reduce the efficiency of these PTs.
An inherent problem with prior thickness mode PTs is that the thickness mode resonant frequency is too high for some applications. Although the high frequency operation initially promotes higher power efficiency, the power loss generated by circulating current in the PT decreases significantly the PT efficiency and consequently increases the heat generation, limiting the maximum power available.
Another problem with prior thickness mode PTs is the losses affecting the driving switching inverter used to drive them, which limit the application of these PTs to high power applications.
Another problem with prior thickness mode PTs is their limitation to reach high output voltages, due to their thin thickness and low output impedance, which leaves them out of the scope of the present invention.
The third family of PTs are the “Laminate-type PTs”. Two types of laminated-type PTs have been so far disclosed in the prior art: Step-down laminated PTs, and Step-up laminated PTs. In the first type, Step-down laminated-type PTs, the input portion (driver section) has a higher impedance (lower capacitance) than the output portion (generator section). Thus, the output voltage of the transformer has a lower value that the input voltage applied to the driver section. Illustrative of such step-down laminated PTs is the device disclosed in U.S. Pat. No. 5,834,882 to Bishop (1998), U.S. Pat. No. 6,333,589 (2001) to Inoi.
This type of Step-down laminated PT suffer of the major drawback that they cannot be used for generating high output voltages, which put them out of the scope of the present invention.
The second category of Laminated PTs correspond to the Step-up Laminated PTs. Illustrative of such laminated PTs is the device of U.S. Pat. No. 6,326,718 (2001) to Boyd, shown in FIG. 5. A laminated PT typically comprises a high impedance portion 60 and a low impedance portion 40 stacked on each other. The remainder of the slab, the vibrator portion 21 is polarized transversely to the plane of the face of the slab (in the thickness direction) and has a high impedance in the direction of polarization. The high impedance portion 60 is polarized in the thickness direction, The high impedance portion 40 is divided in two parts by a belt electrode printed in the center of the slab. Each portion 41 and 42 is polarized in the longitudinal direction.
In the arrangement illustrated in FIG. 5, a voltage applied between the electrodes of the low impedance portion 60 is stepped up to a higher voltage between the electrodes of the high impedance portion 40. The applied voltage causes a deformation of the slab through proportionate changes in the x-y and y-z surface areas. These changes are transmitted toward the high impedance portion 60. A longitudinal vibration is thereby excited in the low impedance vibrator portion 60 in the transverse effect mode (d31 mode). The transverse effect mode vibration in the low impedance vibrator portion 60 in turn excites a vibration in the high impedance generator portion 40 in a longitudinal effect longitudinal vibration mode (g33 mode). As the result, high voltage output is obtained between the electrodes of the high impedance portion 40.
An inherent problem with prior laminated PTs is that the embodiment has a non-symmetric structure with respect to the thickness direction. As a result, several spurious bending resonant modes are excited around the main longitudinal vibration mode. These bending mode diminished the efficiency of the PT.
Another problem of previous laminated PTs is the use of surface belt central electrode in the high impedance section. This surface electrode complicates the polarization process of the samples.
Another problem of the previous laminated PTs is the high level of failure due to the separation of the interface bonding layer joining input and output portions due to the spurious bending modes.
Another problem of the previous laminated PTs is that the acoustic transmission occurs between only one of the major surfaces of the transformer, thus leading to a limitation in the maximum power transmission reachable.
Accordingly, it would be desirable to provide a piezoelectric transformer design that has a higher step-up ratio capacity than similarly sized prior piezoelectric transformers.
It would also be desirable to provide a piezoelectric transformer design that has a higher power transmission capacity than similarly sized prior piezoelectric transformers.
It would also be desirable to provide a piezoelectric transformer that is smaller than prior piezoelectric transformers that have similar power transmission capacities.
It would also be desirable to provide a piezoelectric transformer that is a low profile (height to length ratio) as compared to prior piezoelectric and magnetic transformers that have similar step-up voltage and power transmission capacities.
It would also be desirable to provide a piezoelectric transformer with a symmetric design thereby reducing spurious bending vibration modes.
It would also be desirable to provide a piezoelectric transformer with a significant difference between dimensions in thickness, width and longitudinal dimensions thereby focusing the main resonant vibration mode in the longitudinal direction.
It would also be desirable to provide a piezoelectric transformer in which the “driver” portion of the device and the “driven” portion of the device are not the same electro-active element.
It would also be desirable to provide a piezoelectric transformer that develops a substantial mechanical advantage between the driver portion of the device and the driven portion of the device, thereby making the PT more robust and enhancing the acoustic coupling between input and output sections.
It would also be desirable to provide a piezoelectric transformer having electrode connections that do not effect the resonant operation of the PT.
It would also be desirable to provide a piezoelectric transformer having a configuration minimizing the amount of exposed surfaces having high voltage.
It would be desirable to provide a piezoelectric transformer having a configuration that allow the design of the input and output capacitance with wide freedom.